The invention relates to a web-fed rotary offset printing machine.
Printing machines with multiple motor drives instead of lineshafts are already known from Siemens Zeitschrift 51 (1977) volume 5, pages 387 to 394 and from the journal xe2x80x9cZeitungstechnikxe2x80x9d [newspaper technology], December 1991, pages 78 to 80. In these known printing machines, in each case a plate cylinder is driven by a separate drive motor. The printing mechanisms are already equipped with angle and register control systems, but this prior art does not disclose any possible way in which, with different web paths for different print jobs, the cylinders involved in the printing are to be prepared. Obviously, with the aid of the existing register control system, during printing operation or during transport of the printing-material web, adaptation of the angular positions of the printing cylinders is performed. This gives rise to a high start-up reject rate, since the speed of adjustment during printing can take place only slowly. An excessively high adjustment speed would lead to a web break or to the formation of loops between the printing mechanisms, which is additionally associated with the risk of the web being wound up on one of the printing cylinders. In the case of single-web operation, the cut register can be changed by changing the rotary angle position of the cutting cylinder. During operation, the cut-register control system acts on the rotary angle position of the cutting roll. In the case of multi-web operation, the printing units of all the webs have to be set to the cut register. In a sense, the rotary angle position of the cutting cylinder is the master, to which the angular positions of the individual printing mechanism cylinders have to be adapted. During operation, a cut-register control system ensures the optimum setting of the rotary angle position of the printing cylinders in relation to the cutting cylinder.
During printing operation with optional web guidance, as is needed in particular in newspaper presses, the necessary rotary angle position of the cylinders involved in the printing changes very sharply when the web guidance is changed. German laid-open specification DE-44 30 693 A1 has already disclosed using presetting values with the aid of which, in order to adapt to different web paths and/or production configurations (cut register), presetting of the cylinders involved in the printing is possible for the color register (circumferential register) and the cut register. This results in a simple construction of the machine and/or of the machine control system, and a lower occurrence of rejects in each case when starting up the machine.
On this basis, an object of the present invention is to determine presetting data for the cut register and/or the color register (circumferential register for shaftless printing mechanisms so that, in particular before the beginning of the printing process, optimum presetting is possible by driving the drive motors, without manual interventions or further mechanical adjustments being necessary.
The essence of the invention is that a xe2x80x9cmeasurement of the distancesxe2x80x9d of the respectively activated printing mechanisms or printing points in relation to one another and to the cut edge in the cutting unit is carried out. The respective web length from printing mechanism to cut edge provides the appropriate angle setting. Using an on-line determination (calculation), the respectively applicable or necessary rotary angle position of the printing cylinders is determined, as referred to the current angular position of the cutting cylinder. By means of the individual drive technique, each printing cylinder may then be set individually to the predefined or calculated rotary angle position before the start of the printing process.
Measuring the distances is preferably carried out during the feeding of the printing material or the printing-material web, as follows:
At the unwind, during the feed operation, the paper path is determined continuously, for which purpose corresponding measuring devices are used. A marking, which is applied either to the printing material or to the feeding device, is registered by means of sensors on the printing mechanisms and on the cutting unit. These sensors are fitted at known distances in relation to the individual printing points and the cut edge. During the web feeding operation, the respectively selected printed mechanisms are passed through in the respectively applicable sequence by means of the web feeding device and the printing material. At the same time, the associated web-path distances are read out. In this way, the respectively current distances of the printing mechanisms in relation to one another and to the cut edge at the cutting unit are determined by computation, in each case referred to the web length. If as an alternative, instead of one printing mechanism, a different printing mechanism is used or activated, correspondingly different web lengths apply.
These web lengths can be assigned to the respectively applicable rotary angle positions of these printing cylinders, referred to the rotary angle position of the cutting cylinder, taking into account the circumferential length of the printing cylinders. The feeding of the web is preferably carried out using a web feeding device, as is virtually standard in modern web-fed rotary printing machines. The marking can then be applied both on the feeding device and directly on the printing material, for example in the form of a black adhesive strip which can be read optically.
Following the determination of the web lengths directly after the web feeding, it is usual for the printing material to be tensioned during printing operation, so that the value which is critical for printing operation can deviate slightly from said web lengths. This factor will always exhibit similar values for a selected printing material, so that an advantageous feature of the invention is that this correction can be inserted automatically or taken into account at the same time.
Since the values obtained by this means for the same printing material depend only on the current web value, it is also advantageously possible not only to store these values temporarily as necessary but to store them in the long term and to use them automatically in the case of selected web paths in repeat jobs. In this case, before the beginning of printing, the rotary angle positions of all the printing cylinders involved in the printing process are coordinated with one another and with the cutting cylinder.